Method of preparing photochromic glass

ABSTRACT

A process for producing a photochromic glass which will exhibit a brown coloration in its darkened state without a degradation in the photochromic properties of the glass taking place. The process includes the steps of forming a batch of specified components which include tin or a tin compound and subjecting the resultant batch to a series of glass-forming steps so as to develop photochromic properties in a glass article formed from the molten batch. The invention also provides a photochromic glass having silver halide crystals and tin dispersed throughout the glass, the glass having in its darkened state a brown coloration which is described in accordance with the Hunter L,a,b, Color Scale System by the rectangular region having corners defined by the (a,b) co-ordinates: (5,1), (1,5), (12,16) and (16,12), the 1/2 OD FT of the glass being 120 seconds or less.

FIELD OF THE INVENTION

The present invention relates to photochromic glasses, i.e. to glasscompositions which darken on exposure to actinic radiation and fade backto their original, normally virtually colourless, state when they are nolonger exposed to such radiation.

DESCRIPTION OF PRIOR ART

In our British Patent Specification No. 1,367,903 (equivalent to U.S.Pat. No. 3,876,436), we have described and claimed a range ofphotochromic glasses comprising at least 17% by weight P₂ O₅ as one ofthe glass forming components, with silver halide crystals dispersedthroughout the glass, the total silver content of the glass being atleast 0.05% by weight Ag. The specific glasses disclosed in thatspecification are alumino-phosphate glasses comprising not more than 40%by weight SiO₂ and between 9% and 34% by weight Al₂ O₃ as further glassforming components, and at least 10% by weight R₂ O, where R=K, Na orLi. They can also contain up to 19% by weight B₂ O₃, though most of theglasses disclosed contain no more than 3 to 7% B₂ O₃. Glass compositionsof this type which we have made have had a brown colouration in thedarkened state. However, although the glass compositions exemplified inthe specification of British Pat. No. 1,367,903 exhibit desirablephotochromic properties, they have relatively slow responses to exposureand removal of actinic radiation, i.e. they have relatively slowdarkening and fading rates.

In our British Patent Specification No. 1515642 (equivalent to U.S. Pat.No. 4,092,174) we have described and claimed a range of photochromicglasses having faster responses, particularly a faster fading rate, thanthe glass compositions exemplified in our British Patent SpecificationNo. 1,367,903. The glass compositions of British Patent SpecificationNo. 1,515,642 are photochromic alumino-phosphate glasses having silverhalide crystals dispersed throughout the glass and comprising, asnon-photochromic components in weight percentages:

    ______________________________________                                               SiO.sub.2   8.5 to 25%                                                        Al.sub.2 O.sub.3                                                                          13 to 36.5%                                                       P.sub.2 O.sub.5                                                                           7.5 to 33.5%                                                      B.sub.2 O.sub.3                                                                           7 to 28%                                                          R.sub.2 O   7 to 20.5%                                                 ______________________________________                                    

where R₂ O represents one or more of Na₂ O, K₂ O and Li₂ O, the maximumcontent of Li₂ O being 5%; the amount of SiO₂ is not less than 16% whenthe B₂ O₃ content is less than 8%; and, as photochromic components,expressed as weight percentages over and above the 100% total of all thenon-photochromic components of the glass:

silver, expressed as Ag₂ O not less than 0.05%

Cl+Br 0.20 to 2%.

When heat treated to develop optimum photochromic properties asdiscussed in the above-mentioned patent, these glasses have been foundto have a good combination of induced optical density on irradiationwith actinic light and rapid darkening on irradiation and rapid fadingwhen irradiation ceases, and to have a grey colouration in theirdarkened state. These and other presently-available fast responsephotochromic glasses utilized for ophthalmic and other applicationsexhibit a grey colouration upon activation with actinic radiation. Thecolour of these glasses is a function of the composition utilized toproduce the photochromic glass, and of the thermal treatment utilized toconvert the potentially-photochromic glass as formed into a photochromicarticle exhibiting the desired sensitivity to actinic light.

Whereas prior art fast response photochromic glasses for ophthalmic andother uses have been widely available in a grey colouration in thedarkened state, there is a demand for photochromic glasses of equivalentsensitivity and behaviour, but exhibiting other colourations such asbrown.

A number of methods for converting photochromic glasses which exhibit agrey colouration in the darkened state to a glass which exhibits a browncolouration in the darkened state have been proposed. For example, inone method a colourant which is compatible with the photochromicconstituents in the glass is added to the base glass. Thus, for example,German Offenlegungschrift No. 2,107,343 describes photochromic glassescontaining additions of vanadium, chromium, manganese and cobalt, andthe resultant glasses are said to exhibit a variety of colours incombination with photochromic properties.

It has also been suggested to subject photochromic glasses to an"after-treatment" by heating completely developed photochromic glassesin a reducing atmosphere to impart a yellow or brown colourationthereto. Such methods are described in U.S. Pat. Nos. 3,892,582 and3,920,463.

In a third known method the number and size of silver halide particlesin certain specified boro-silicate glass compositions are modified bysubjecting the glasses to a two phase-heat treatment procedure. Such amethod is described in U.S. Pat. No. 4,043,781.

U.S. Pat. No. 4,043,781 gives an indication of some of the problemspresented by the first two above mentioned known methods. In the firstmethod, the addition of a colourant to the base glass leads to areduction in the level of transmission in the faded state. Furthermoreany colour is superimposed on the normal grey appearance of the glass.The method in which an "after-treatment" is used requires a furtherprocessing step in the preparation of the glass which can addsubstantially to the cost of the final glass product. It has also beenfound that such an "after-treatment" provides predominantly surfacecolouration. Unless the after-treatment is applied to a finished lens,the colouration of different areas of the lens will vary as the lensblank is ground and polished to form a finished lens.

The method disclosed in U.S. Pat. No. 4,043,781 provides certainadvantages over the first two said known methods. For example, no addedcolourant needs to be used, and the brown colouration is producedthroughout the bulk of the glass article. Thus, in the case of theboro-silicate glasses described in U.S. Pat. No. 4,043,781 it is said tobe possible to achieve a brown colouration throughout the entire volumeof a photochromic glass article by a two stage heat treatment, withoutany material change in the darkening and fading rates of the glass. Inattempting to apply the teaching of U.S. Pat. No. 4,043,781 to thephotochromic glasses of British Patent Specification No. 1,515,642, wehave found that while modification of the heat treatment schedules canresult in a glass which exhibits a brown colour in the darkened stage,such modified heat-treatment also produces deterioration in photochromicproperties.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide a process for producinga photochromic glass which will exhibit a brown colouration in itsdarkened state without a degradation in the photochromic properties ofthe glass taking place.

According to a first aspect of the present invention there is provided aprocess for producing a photochromic glass containing silver halide andwhich has a brown colouration in its darkened state, which processcomprises (a) forming a batch comprising tin or a tin compound in anamount which will produce a glass which will exhibit a brown colourationin its darkened state, together with those glass forming componentswhich, in the absence of tin or the said tin compound and when subjectedto steps (b) to (f) below, will produce a photochromic glass having agrey colouration in its darkened state, (b) melting the batch to form amolten glass, (c) refining the molten glass, (d) conditioning the moltenglass to bring it into a state suitable for forming into articles, (e)forming the molten glass into articles, and (f) subjecting the formedarticles to a heat-treatment which will develop the photochromicproperties of the glass, steps (b) to (f) being performed underconditions which, in the absence of tin or the said tin compound, willproduce a glass having a grey colouration in its darkened state.

According to a further aspect of the present invention there is provideda photochromic glass having silver halide crystals and tin dispersedthroughout the glass, the glass having in its darkened state a browncolouration which is described in accordance with the Hunter L,a,b,Colour Scale System by the rectangular region having corners defined bythe (a,b) co-ordinates: (5,1), (1,5), (12,16) and (16,12), the 1/2 OD FTof the glass being 120 seconds or less.

DETAILED DESCRIPTION

The brown colouration which the glass product exhibits in its darkenedstate can be regulated by the quantity of tin or tin compound added andby adjustment of the melting conditions in steps (b) and (c) and/or ofthe heat-treatment conditions of step (f).

Steps (b) to (f) are usually performed in conventional manner and usingprocess conditions which are known to produce a photochromic "grey"glass, that is to say a glass which exhibits a grey colouration in thedarkened state.

Preferably, the process of the present invention is performed incontinuous manner.

In step (a) those components which will produce a photochromic glasshaving a grey colouration in its darkened state can be, for example,those components used to prepare the photochromic glass compositionsdescribed and claimed in our British Patent Specification No. 1,515,642(The disclosure of this British Patent is incorporated herein byreference). These known fast response glass compositions arephotochromic aluminophosphate glasses having silver halide crystalsdispersed throughout the glass, the glass comprising as nonphotochromiccomponents in weight percentages:

    ______________________________________                                               SiO.sub.2   8.5 to 25%                                                        Al.sub.2 O.sub.3                                                                          13 to 36.5%                                                       P.sub.2 O.sub.5                                                                           7.5 to 33.5%                                                      B.sub.2 O.sub.3                                                                           7 to 28%                                                          R.sub.2 O   7 to 20.5%                                                 ______________________________________                                    

where R₂ O represents one or more of Na₂ O, K₂ O and Li₂ O, the maximumcontent of Li₂ O being 5%; the amount of SiO₂ is not less than 16% whenthe B₂ O₃ content is less than 8%; and, as photochromic components,expressed as weight percentages over and above the 100% total of all thenon-photochromic components of the glass:

    ______________________________________                                        silver, expressed as Ag.sub.2 O                                                                   not less than 0.05%                                       Cl + Br             0.20 to 2.0%                                              ______________________________________                                    

Glass compositions of this type may be made up in the following manner:A batch of the glass-forming components is melted and refined underoxidising or neutral conditions at a temperature in the range 1200° C.to 1600° C. The glass is then conditioned, formed into articles, andafter cooling is annealed at a temperature between 450° C. and 650° C. Afinal heat-treatment which has the effect of developing the photochromicproperties of the glass may subsequently be effected at between 20° and100° C. above the annealing point for a period of 10 to 60 minutes. Theoptimum heat-treatment temperature range for a particular glass may bedetermined by a gradient furnace technique. In some cases, it may benecessary to support the glass during heat-treatment to avoid sagging.The batches for making the glass can be made up from conventionalglass-making raw materials, such as carbonates, meta- or orthophosphates, nitrates and oxides. The silver and halide components may beadded to the batches in the form of finely-ground silver salts andsodium or potassium halides, respectively.

Precautions are required during melting to minimise volatilisationlosses of batch components. Up to 60% by weight of the halide componentsand 30% by weight of silver may be lost in this way and the necessaryallowances are required during batch preparation.

As has been stated, the photochromic effect of the glass compositions ofour British Pat. No. 1,515,642 can be developed by heat-treatment of theglass, the appropriate heat-treatment schedule being primarilydetermined by the viscosity-temperature relationship of the particularglass. In general, the heat-treatment temperature lies between thestrain point and the softening point of the glass, the heat-treatmenttime required being several hours at the lower temperature but only afew minutes at the higher temperature. At the higher temperature,however, deformation and clouding of the glass may occur, so it ispreferred for convenience to use a temperature 20° to 100° C. above theannealing point and a heat-treatment time of 10 to 60 minutes. Theschedule may be imposed on the glass directly after forming or the glassmay be annealed and cooled to room temperature before heat-treatment.The cooling rate to which the glass is subjected after heat-treatment issometimes found to have an effect on the photochromic properties of thefinal product. This cannot be stated as a general rule, however, andmust be determined by experimentation on individual glasses. Thetemperature/time schedule imposed on a glass is also determined by theconcentrations of photochromic agents in the glass and the photochromicproperty requirements of the final product. In general, the higher thelevels of the components contributing to the photochromism the shorterwill be the heat-treatment schedule, and in some cases, thephotochromism may develop during cooling from the melt or annealing ofthe glass. Excessively long heat treatments are generally to be avoidedbecause they will lead to some clouding of the glass. These comments onthe subsequent heat-treatment of the glasses of British Pat. No.1,515,642 are equally applicable to the heat-treatment of step (f) ofthe present process.

We have found that if one subjects a fast response photochromic glass ofthe type described and claimed in our British Patent Specification No.1,515,642 to a two-stage heat-treatment of the type described in U.S.Pat. No. 4,043,781, one can obtain a glass having a brown colouration inits darkened state. However this two-stage heat treatment has theunfortunate effect of also producing a deterioration in the photochromicproperties of the glass.

In contrast, in accordance with the process of the present invention, byadding a small proportion of tin or a tin compound to a batch comprisingthe components of the above-mentioned fast-response photochromic glasscompositions, one can obtain a photochromic glass which has a browncolouration in its darkened state without any accompanying degradationof the photochromic properties of the glass. In addition, havingselected a preferred brown colouration for the glass in its darkenedstate, one can, if desired, control the amount of tin or tin compoundadded to the batch and regulate the process conditions so as to obtain,as the product of the process, a fast response photochromic glass havingthe selected brown colouration in its darkened state.

It is believed that, under the melting conditions in a glass meltingtank, tin or any tin compound added will generate a proportion of tin ina reduced ionic state in the glass. It is further believed that thisreduced form of tin acts as a reducing agent which enables the browncolouration in the darkened state of the glass to be obtained.

It has previously been considered that it is usually detrimental to havereducing agents or reducing conditions present during the formation of aphotochromic glass. The reason for this is that in some circumstancesthe silver halides present in the glass can be reduced to metallicsilver giving either silver balls in the finished glass or a silver rubyglass with no or poor photochromic properties. This is still so, but wehave found that by adding a controlled amount of tin or a tin compoundit is possible to obtain the advantageous formation of a photochromicglass which assumes a brown colouration in its darkened state withoutadversely affecting the photochromic properties of the glass. It iscommercially desirable to have both brown and grey glasses available andthe process of the present invention enables one to move easily from oneof said colours to the other during a production campaign by adding orceasing to add tin or a tin compound to the batch of glass-formingcomponents.

Tin or any tin-containing compound which will generate, under theconditions of the present process, tin in a reduced ionic state in theglass can be used in the process of the present invention. It ispreferred that the tin compound is one which is physically compatiblewith the batch, that is to say that it is a solid, which is preferablyin finely divided form. Furthermore the tin compound is preferably anon-toxic material, is "stable" in the sense that it gives no losses orinsignificant losses of tin during melting of the batch, and is amaterial which does not affect the chemical properties of the resultantglass product. Desirably the tin compound is one which is readilyavailable and is of low cost. If a relatively volatile tin compound isused, the amount of tin compound used must be such as to compensate forthe losses of compound which result from volatilisation.

If it is desired to add metallic tin to the batch, the tin must be in aform which is such that silver spheres and other inclusions will not beformed in the molten glass.

Stannous oxide and stannic oxide are examples of preferred tin compoundsfor use in the present invention.

The quantity of tin or tin compound to be included in the batch ofglass-forming components can be easily determined by a man practiced inthe art. It will be dependant to some extent on the throughput, i.e. thetime at which the glass is held at a particular temperature, and on themelting and heat treatment conditions being used with a particular glasscomposition.

Preferably the amount of tin or tin compound added is such as to producea finished glass which in its darkened state has a brown colourationwhich is described in accordance with the Hunter L,a,b Colour Scalesystem (see "Measurement of Appearance" by R. S. Hunter, pages 122 and123) by the rectangular region on the a,b-plane of the Hunter systemwith corners on the (a,b) coordinates: (5,1), (1,5), (12,16) and(16,12). By way of comparison, the grey colouration of glasses discussedin this specification is usually a colour which can be described by anarea on the a,b-plane of the Hunter L,a,b system which is aquadrilateral having corners at the (a,b)-coordinates: (3.5,2),(7,-5.5), (-3.5,-5.5), and (-3.5,2). Glasses having a colour in thisarea can be said to be substantially grey in appearance.

The above specific colours are defined with reference to the C.I.E.illuminant C (see page 50 of "Measurement of Appearance" by R. S.Hunter).

The effect of increasing the level of tin or tin compound in the glassbatch without any other change in conditions is to make the glass assumea more intense brown colouration in its darkened state, i.e. the values(a,b) in the Hunter L,a,b, co-ordinate system increase. If desired, theglass can be made less brown in colour again by reducing the quantity oftin or tin compound. It is not usual when operating a glass meltingprocess to alter the throughput of glass once this has been fixed, butany change in throughput will change the time the molten glass has beenkept at particular temperatures. Accordingly, should a change inthroughput occur, a change in tin or tin compound level will be needed.

In giving guidance as regards appropriate levels of tin or tin compoundto add to the batch, it is impossible to state that a particularquantity should be used with a particular glass batch, as there must becorrelation with melting temperature, throughput, heat-treatmentconditions and annealing schedules. Thus in general we find that havingset a target colour based on the Hunter L,a,b, system, a quantity of tinor tin compound is added to the batch of glass-forming components, andthe variance from target colour of the resultant glass is determined.Thereafter, the amount of tin or tin compound and/or process conditionsare adjusted as necessary until the finished glass will give the desiredtarget colour. These changes or the initial addition of tin or tincompound will only cause a deterioration in photochromic properties ifthe quantity of tin or tin compound is excessive and causes thetransformation of a large proportion of the silver present to elementalsilver either giving a silver ruby or small discrete silver spheres.

In general, we have found that an amount of tin or tin compound whichwill produce 500 to 5000 ppm, preferably 500 to 3500 ppm of tin,expressed as SnO₂, in the finished glass is suitable.

Some fine adjustment of the colour exhibited by the glass in itsdarkened condition may be achieved by altering the conditions underwhich the glass is heat-treated and annealed.

In some cases it may be found desirable to add reducing agents, such asfor example starch or potassium hydrogen tartrate, to the glass batch soas to increase the proportion of tin in the reduced ionic state relativeto the amount of tin in a non-reduced or less reduced ionic state. Whensuch additional reducing agents are added, it is important to ensurethat the amount and form of such agents is not such as to causeelemental silver in the shape of small spheres to separate in the melt.

We do not know the exact reason for our success in obtaining aphotochromic glass having a brown colouration in its darkened conditionwithout any falling off in photochromic response when compared to aphotochromic glass of a similar composition which has a grey colourationin its darkened state. One theory is that a very small proportion of thetin is in a particular ionic state which enables it to convert a verysmall proportion of dissolved silver ions into a silver precipitate.This precipitate is so fine and well distributed that its individualparticles act as nucleating agents on which the silver halide crystalsgrow, thus giving a proportion of free silver in the silver halideparticles. This in turn could alter the sites on which silver isproduced photo-chemically during darkening giving a different darkenedcolour. Thus we believe a reduced ionic state of tin is having an effectduring heat-treatment rather than during the melting process.

The process of the present invention can also be used with batches ofglass-forming components which include a colouring agent which producesa tint such as, for example, amber in the glass when in its fadedcondition.

In a preferred embodiment of the process of the present invention, inorder to produce in a continuous process glass having a satisfactorycolour in its darkened condition, one first produces a glass of acomposition which will produce the desired photochromic properties. Oncethe glass batch has been adjusted to suit the particular meltingconditions and a suitable heat-treatment schedule has been established,i.e. so that for example the photochromic glass which is produced is onehaving 16% transmission in its darkened state and 88% transmission inits clear state with a 1/2 OD FT of 45 seconds, tin or a tin compound isadded to the batch at a level such as to give for example 5000 ppm tin,expressed as SnO₂, in the finished glass. The colour of the finishedglass in the darkened condition is measured and then the level of tin ortin compound in the batch can be raised or lowered to obtain a glasshaving a desired brown colouration in its darkened condition, forexample brown colour in the region defined by coordinates (5,1), (1,5),( 12,16) and (16,12) in the Hunter L,a,b, Colour Scale system. We havefound that it is generally possible to obtain a given brown colourationwith a range of levels of tin in the finished glass. Thus, for example,in cases where a particular brown colouration can be obtained withlevels of 3000 ppm of tin, expressed as SnO₂, in the finished glass, itmay be possible to obtain the same brown colouration when the quantityof tin is reduced to a level of the order of 900 to 1000 ppm in thefinished glass. This may be due to the possibility that under steadyconditions no matter how much tin or tin compound is added, the quantityof tin in a reduced ionic state which is "active" to produce the browncolour is of the same order.

According to a further aspect of the present invention there is provideda photochromic glass having silver halide crystals and tin dispersedthroughout the glass, the glass having in its darkened state a browncolouration which is described in accordance with the Hunter L,a,b,Colour Scale system by the rectangular region having corners defined bythe (a,b) co-ordinates: (5,1), (1,5), (12,16) and (16,12), the 1/2 OD FTof the glass being 120 seconds or less.

Preferred such glasses have a composition as claimed in our BritishPatent Specification No. 1,515,642 and include tin in the finished glasscomposition.

Preferably the amount of tin in the finished glass, expressed as SnO₂,is 500 to 5000 ppm, more preferably 500 to 3500 ppm.

In this specification the property of a glass composition referred to as1/2 OD FT is the time in seconds taken to fade to a condition of halfthe total induced optical density, measured with standard samples ofglass 2 mm thick at 25° C., in standard simulated solar conditions atair mass 2 (see Parry Moon, J. Franklin Inst., 230 (1940), pages583-617). The induced optical density is the difference between theoptical density of the glass in the fully darkened state and the opticaldensity in the fully faded state, the optical density being defined inthe conventional manner as

    log 10(I.sub.i /I.sub.t),

where I_(i) is the intensity of the incident light and I_(t) is theintensity of the transmitted light. The induced optical density is thusa real measure of the photochromic effect and is in fact directlyproportional to the number of photochromically activated silver atoms ina given volume of the glass. The time required to fade from the fullydarkened condition to a condition of half the induced optical density(1/2 OD FT) is thus an effective measure for comparing fading times ofglasses having different values of light transmission in the bleached orfaded state.

The fully darkened state of the glass compositions of the presentinvention is defined as the state reached by a standard sample ofphotochromic glass 2 mm thick at 25° C. after exposure to theabove-mentioned standard simulated solar conditions at air mass 2 for 23minutes.

The following Examples illustrate the present invention.

EXAMPLES

Two glasses (compositions I and II) having compositions closely similarin all respects except that composition I included no tin, whilecomposition II included a small proportion of added SnO₂, were preparedby melting a batch of appropriate glass-forming components at thetemperatures and times shown in Table I. The glass compositions werethen processed in accordance with the conditions given in Table I, andthe colour and photochromic properties of flat polished samples 2 mmthick made from the resultant compositions were measured.

For each of compositions I and II the base glass composition on theoxide basis in weight percentages was:

    ______________________________________                                               P.sub.2 O.sub.5                                                                            15.2                                                             Al.sub.2 O.sub.3                                                                           27.6                                                             SiO.sub.2    17.7                                                             CaO          2.9                                                              BaO          9.0                                                              TiO.sub.2    0.5                                                              ZrO.sub.2    0.8                                                              K.sub.2 O    10.9                                                             Li.sub.2 O   1.5                                                              B.sub.2 O.sub.3                                                                            13.9                                                             Co.sub.3 O.sub.4                                                                           16     ppm.                                               ______________________________________                                    

In addition, glass composition II includes 1400 ppm of tin, expressed asSnO₂ in the finished glass.

From the data given in Table I, it will be seen that glass compositionII has a brown colouration in its darkened state as manifest by Huntera,b-co-ordinates (3.9, 3.1), while glass composition I, which has notin, has a grey colouration in its darkened state as manifest by Huntera,b-co-ordinates (2.8-2.1).

From a consideration of the transmission of glass compositions I and IIin their darkened and faded conditions and their D_(x),F_(x) values (seeTable I) it will be seen that the presence of tin in the glass has notadversely affected the photochromic properties of the glass.

Examples of some further glass compositions in accordance with theinvention are given in Table II. The base glass composition of theseglasses was very similar to that of the base glass composition of GlassII. These glasses show a more intense brown colouration in theirdarkened state than glass composition II, and yet retain desirable fastresponses to exposure to and removal from actinic radiation as ismanifest by the D_(x),F_(x) and 1/2 OD FT values set out in Table II.

Glasses III, IV and V contained respective amounts of tin (analysed asSnO₂) of 2,100 ppm, 3,000 ppm and 890 ppm in the finished glass.

                  TABLE I                                                         ______________________________________                                                        I                                                                             (Comparative)                                                                             II                                                Glass Composition                                                                             ("Grey Glass")                                                                            ("Brown Glass")                                   ______________________________________                                        Photochromic components.sup.7                                                 Ag.sub.2 O      0.396       0.320                                             CuO             0.041       0.036                                             Cl              0.60        0.35                                              Br              0.14        0.30                                              Process conditions                                                            Melting Temperature/Time                                                                      1300° C./3 hr                                                                      1350° C./3 hr                              Refining Temperature/Time                                                                     1400° C./2 hr                                                                      1400° C./2 hr                              Heat-treatment of Step                                                                        655° C./16 min                                                                     655° C./16 min                             (f)/Time                                                                      Optical Properties                                                            % Transmission in                                                                             87%         87%                                               Faded State                                                                   % Transmission in                                                                             19.3%       24.1%                                             Darkened State                                                                Colour in Faded State.sup.2                                                                   (-1.5, 2.9) (-2.3, 4.2)                                       Colour in Darkened State.sup.2                                                                (2.8, -2.1) (3.9, 3.1)                                        D.sub.x=8.spsb.3                                                                              43.3%       52.3%                                             D.sub.x=60.spsb.3                                                                             74.5%       79.3%                                             F.sub.x=16.spsb.3                                                                             33.9%       48.1%                                             F.sub.x=600.spsb.3                                                                            87.5%       93.4%                                             1/2 OD FT       approx 45 secs                                                                            20-25 secs.                                       ______________________________________                                    

                  TABLE II                                                        ______________________________________                                        Glass Composition                                                                            III       IV        V                                          ______________________________________                                        Photochromic components.sup.7                                                 Ag.sub.2 O      0.362    0.33      0.37                                       CuO            0.30      0.30      0.38                                       Cl             0.23      0.25      0.16                                       Br              0.027     0.027     0.026                                     Process conditions                                                            Melting Temperature                                                                          1260° C.                                                                         1300° C.                                                                         1340° C.                            Refining Temperature                                                                         1390° C.                                                                         1450° C.                                                                         1500° C.                            Heat-treatment of step (f)                                                                    627° C.                                                                          655° C.                                                                          677° C.                            Optical Properties                                                            % Transmission in                                                                            88.6%     84.1%     87.1%                                      Faded State                                                                   % Transmission in                                                                            25.5%     14.4%     15.7%                                      Darkened State                                                                Colour in Faded State.sup.2                                                                  (-4.3, 7.5)                                                                             (-2.8, 7.7)                                                                             (-2.3, 4.5)                                Colour in Darkened State.sup.2                                                               (5.6, 4.3)                                                                              (7.6, 7.6)                                                                              (7.2, 6.4)                                 D.sub.x=8.spsb.3                                                                             40.9%     38.4%     42.1%                                      D.sub.x=60.spsb.3                                                                            85.8%     73.5%     75%                                        F.sub.x=16.spsb.3                                                                            47.2%     34.4%     31%                                        F.sub.x=600.spsb.3                                                                           86.4%     86.7%     88.4%                                      1/2 OD FT      approx 25 approx 45 approx 50                                                 secs      secs      secs.                                      ______________________________________                                    

Key to Tables I and II

1. Measured as weight percentages over and above the 100% total of allthe non-photochromic components of the glass (i.e. the base glass)

2. Colour expressed as (a,b) co-ordinates in the a,b-plane of the HunterL,a,b, Colour Scale System. Colour measured by standard techniques.

3. D_(x) and F_(x) represent the percentage of the total change inoptical density which occurs after darkening (D) or fading (F) for atime of x seconds.

We claim:
 1. A process for producing a photochromic glass containingsilver halide and which has a brown colouration in its darkened state,which process comprises (a) forming a batch comprising tin or a tincompound in an amount which will produce a glass which will exhibit abrown colouration in its darkened state, together with those glassforming components which, in the absence of tin or the said tin compoundand when subjected to steps (b) to (f) below, will produce aphotochromic glass having a grey colouration in its darkened state, (b)melting the batch to form a molten glass, (c) refining the molten glass,(d) conditioning the molten glass to bring it into a state suitable forforming into articles, (e) forming the molten glass into articles, and(f) subjecting the formed articles to a heat-treatment which willdevelop the photochromic properties of the glass, steps (b) to (f) beingperformed under conditions which, in the absence of tin or the said tincompound, will produce a glass having a grey colouration in its darkenedstate.
 2. A process according to claim 1, wherein the process isperformed in continuous manner.
 3. A process according to claim 1,wherein the components used in step (a) are components which, in theabsence of tin or the said tin compound, will produce a fast responsephotochromic alumino-phosphate glass having silver halide crystalsdispersed throughout the glass and having a grey colouration in itsdarkened state, the glass comprising as non-photochromic components inweight percentages:

    ______________________________________                                               SiO.sub.2   8.5 to 25%                                                        Al.sub.2 O.sub.3                                                                          13 to 36.5%                                                       P.sub.2 O.sub.5                                                                           7.5 to 33.5%                                                      B.sub.2 O.sub.3                                                                           7 to 28%                                                          R.sub.2 O   7 to 20.5%                                                 ______________________________________                                    

where R₂ O represents one or more of Na₂ O, K₂ O and Li₂ O, the maximumcontent of Li₂ O being 5%; the amount of SiO₂ being not less than 16%when the B₂ O₃ content is less than 8% and, as photochromic components,expressed as weight percentages over and above the 100% total of all thenon-photochromic components of the glass:

    ______________________________________                                        silver, expressed as Ag.sub.2 O                                                                   not less than 0.05%                                       Cl + Br             0.20 to 2.0%.                                             ______________________________________                                    


4. A process according to claim 1, 2 or 3, wherein the tin compound isstannous oxide or stannic oxide.
 5. A process according to claim 1, 2 or3, wherein the tin compound is in the form of a finely divided solid. 6.A process according to claim 1, 2 or 3, wherein the amount of tin or tincompound added is such as to produce a finished glass which in itsdarkened state has a brown colouration which is described in accordancewith the Hunter L, a,b Colour Scale system by the rectangular region onthe a,b-plane of the Hunter system with corners on the (a,b)co-ordinates: (5,1), (1,5), (12,16) and (16,12).
 7. A process accordingto claim 1, wherein the amount of tin or tin compound used is an amountwhich will produce 500 to 5000 ppm, expressed as SnO₂, in the finishedglass.
 8. A process according to claim 7, wherein the amount of tin ortin compound used is an amount which will produce 500 to 3500 ppm,expressed as SnO₂, in the finished glass.
 9. A process according toclaim 1, wherein the batch of components in step (a) includes one ormore reducing agents.
 10. A process according to claim 9, wherein thereducing agent is starch or potassium hydrogen tartrate.
 11. A processaccording to claim 1, wherein the batch of glass-forming components instep (a) includes a colouring agent which produces a tint in the glasswhen the glass is in its faded condition.
 12. A process according toclaim 11, wherein the tint is amber.
 13. A photochromic glass havingsilver halide crystals and tin dispersed throughout the glass, the glasshaving in its darkened state a brown colouration which is described inaccordance with the Hunter L,a,b Colour Scale System by the rectangularregion having corners defined by the (a,b) co-ordinates: (5,1), (1,5),(12,16) and (16,12), the 1/2 OD FT of the glass being 120 seconds orless, the glass being an alumino-phosphate glass having silver halidecrystals dispersed throughout the glass and comprising, asnon-photochromic components in weight percentages:

    ______________________________________                                               SiO.sub.2   8.5 to 25%                                                        Al.sub.2 O.sub.3                                                                          13 to 36.5%                                                       P.sub.2 O.sub.5                                                                           7.5 to 33.5%                                                      B.sub.2 O.sub.3                                                                           7 to 28%                                                          R.sub.2 O   7 to 20.5%                                                 ______________________________________                                    

where R₂ O represents one or more of Na₂ O, K₂ O and Li₂ O, the maximumcontent of Li₂ O being 5%; the amount of SiO₂ being not less than 16%when the B₂ O₃ content is less than 8%; and, as photochromic components,expressed as weight percentages over and above the 100% total of all thenonphotochromic components of the glass:

    ______________________________________                                        silver expressed as Ag.sub.2 O                                                                    not less than 0.05%                                       Cl + Br             0.20 to 2%,                                               ______________________________________                                    

the finished glass composition further including tin.
 14. A photochromicglass according to claim 13, wherein the amount of tin in the finishedglass, expressed as SnO₂, is 500 to 5000 ppm.
 15. A photochromic glassaccording to claim 14, wherein the amount of tin in the finished glass,expressed as SnO₂, is 500 to 3500 ppm.